Algal Blooms in the Baltic: When Not to Swim

Every Swedish summer the Baltic turns green in places, and most of the time the rest of the coast is perfectly swimmable. The trick is knowing which water is which. Here’s why it happens, where to check, and what to do if someone swims in it anyway.

When you need this

You’re heading for the east coast, the Stockholm archipelago, Öland or Gotland in July or August. You’ve reached a beach and the water looks green, streaky or like thin soup, or there’s a notice advising against swimming. Maybe your dog is already wading in, or your island cabin draws its water from the sea — or someone feels sick a few hours after a swim.

Why the Baltic blooms every summer

What Swedes call algblomning (algal bloom) is mostly cyanobacteria, microscopic organisms once known as blue-green algae. In the Baltic’s brackish water two species dominate. Aphanizomenon flos-aquae makes no toxin, though it can still form harmful mass blooms. Nodularia spumigena — katthårsalg, “cat’s-hair alga” — produces the liver toxin nodularin, which has so far only been found in the Baltic.

The mechanism is simple. After the spring bloom, the plankton has used up the nitrogen in the water, but phosphorus is often left over. Cyanobacteria can fix nitrogen gas dissolved from the air, so at that moment they have the water to themselves. Warm water and sunny, calm weather make the blooms extra large, and in still spells some cyanobacteria float up and form dense scum on the surface.

That’s why the bloom is a certainty, not a freak event: there are blooms in the Baltic every summer. What varies is how much of it gathers on the surface — more in sunny summers with light winds, less in windy, cool ones.

Summer 2026 shows the rhythm:

How to check before you swim

Start with SMHI’s map. The Swedish Meteorological and Hydrological Institute runs a free service called Algsituationen – cyanobakterier at smhi.se/algsituationen. In summer it publishes a map of the Baltic every day, interpreted by an expert from that day’s satellite images and updated the same afternoon. SMHI has mapped blooms this way since 2002.

Reading it takes ten seconds:

On cloudy days, switch to the weekly summary tab. It shows on how many of the last seven days a bloom was seen in each area. If a bloom was there before the clouds rolled in, it’s probably still there.

Know what the map can’t do. It sees only what has gathered at or near the surface, it sees nothing through cloud or in darkness, and not every colour change is cyanobacteria — pollen and stirred-up sediment can look similar. Above all, SMHI’s own oceanographers say you can’t tell whether a bloom is toxic from a satellite image or with the naked eye. The service runs in summer only.

For local detail on the east coast, the county boards run marine information centres — one for the Baltic Proper, one for the Gulf of Bothnia — that publish summer reports on the algläget (algae status) and take reports and photos of blooms from the public.

Then check the notice at the bathing place. Municipalities must check their official bathing waters for cyanobacteria and must avråda — advise against swimming — when a situation threatens health. It’s advice, not a ban: nobody will fine you, and nobody will stop you. How the bathing-water system works is covered in our guide to swimming in Swedish lakes.

What a bloom looks like at the water’s edge

1177, Sweden’s official health advice service, describes a blue-green, green or yellow-green film on the surface, water made cloudy by small thin threads or grains that look like pine needles, and a changed smell and taste. For the visibility test — can you see your feet? — see the lake swimming guide.

Three things catch visitors out:

• A bloom isn’t always on the surface. It can sit several metres down, which is why SMHI marks some areas blue.
• Clear water after a bloom isn’t a green light. Toxin can linger after the algae go; 1177 advises waiting a few days before swimming in sheltered coves.
• Washed-up algae mats are toxic too. Don’t touch them, and keep children and dogs off them.

The west coast isn’t immune either. The Baltic species can’t grow in the saltier Kattegat and Skagerrak, but winds and currents can carry toxic blooms there. Lakes bloom too, from south to north, mostly with a different toxin group (microcystins).

If you swam in it: symptoms and who to call

According to 1177, symptoms usually start a few hours after swimming:

• rash and itching
• red or itchy eyes
• headache
• stomach illness with nausea, vomiting and diarrhoea

With large amounts, fever and muscle weakness can follow, and the liver and kidneys can be affected. The stomach illness usually lasts a day or a few. There’s no specific treatment; care treats the symptoms.

Children are the most exposed, because children playing at the water’s edge swallow a lot of water — and 1177 notes you can fall ill from small amounts in a heavy bloom.

Right after an accidental swim: shower, soap your whole body and rinse with clean water. Rinse your eyes thoroughly if they sting.

Who to call:

More on reaching care as a visitor is in our guide to how 1177 works.

Dogs and algae

Dogs that swim often and for a long time and drink the water run a particular risk, and 1177 calls dogs, cats and other pets especially sensitive. Signs — vomiting, drooling, diarrhoea, unsteady gait, itching and irritated eyes — can start within hours. Paralysis and convulsions often come quickly, and animals die.

Beached bloom is the other trap: a dog that licks washed-up remains can suffer serious internal bleeding in the liver.

If your dog has been in it, rinse its coat and eyes with clean water and call a vet. Don’t call the poison centre — Giftinformationscentralen hasn’t answered questions about animals since 1 July 2018. See vets in Sweden and our guide to dogs in Swedish nature, which also covers the leash rule running 1 March to 20 August.

Drinking water: boiling does not help

This is where the usual outdoor rule fails. Algal toxins are stable, and boiling doesn’t destroy them — Livsmedelsverket (the Swedish Food Agency), Folkhälsomyndigheten and 1177 all say so. Livsmedelsverket’s advice is never to drink lake water straight from the lake.

1177 adds that a desalination unit doesn’t remove the toxin either. That matters on islands, where some cabins make their water from the sea. Municipal tap water is a different matter: waterworks run treatment steps that reduce and remove cyanobacteria and their toxins. At a cabin with its own supply, ask the host where the water comes from. See tap water in Sweden.

Fishing and eating fish during a bloom

Livsmedelsverket’s food advice is short and specific:

• Don’t eat the liver of burbot (lake in Swedish — a fish, not a lake) caught in water with a cyanobacterial bloom.
• Don’t eat fish found dead or sick.
• Don’t eat large amounts of crayfish from water with a toxic bloom — relevant to crayfish fishing in August.

SMHI adds that algal toxins can build up in shellfish such as mussels and oysters.

Practical information

FAQ

Is it safe to swim in the Baltic Sea in summer?

Usually, yes — but not in a bloom. Cyanobacteria bloom in the Baltic every summer, strongest in warm, calm weather from July. Stay out of water with a green or yellow-green film, streaks, threads or grains, and keep children and dogs away. You cannot tell a toxic bloom from a harmless one by eye, so treat every visible bloom as toxic.

How can I check for algal blooms in Sweden?

SMHI publishes a free map, Algsituationen – cyanobakterier, at smhi.se/algsituationen, updated daily in summer from satellite images. Green shows scum at the surface, blue below it, grey is cloud. It cannot see through cloud or show toxicity, so also look at the water itself and read any municipal notice at the beach.

What are the symptoms of algae poisoning?

According to 1177, symptoms usually start a few hours after swimming: rash and itching, sore or itchy eyes, headache, and nausea, vomiting and diarrhoea. Large amounts can cause fever, muscle weakness and liver or kidney effects. The stomach illness usually lasts a day or a few. There is no specific treatment. Call 1177 for advice, or 112 in an emergency.

Can my dog swim in the Baltic during an algal bloom?

No. Dogs that swim often and drink the water run a particular risk, and 1177 calls pets especially sensitive. Symptoms such as vomiting, drooling and unsteady gait can start within hours; severe cases bring paralysis and convulsions, and dogs die. Keep the dog off washed-up algae too, rinse its coat with clean water and call a vet.

Can you eat fish caught during an algal bloom?

Livsmedelsverket gives three specific rules: do not eat the liver of burbot (lake) caught in water with a cyanobacterial bloom, do not eat fish found dead or sick, and do not eat large amounts of crayfish from water with a toxic bloom. SMHI adds that algal toxins can build up in mussels and oysters.

Can I drink lake or sea water if I boil it?

Not during a bloom. The toxins are stable and boiling does not destroy them; Livsmedelsverket says never to drink lake water straight from the lake. 1177 adds that a desalination unit does not remove the toxin either. Municipal tap water is treated at waterworks; at a cabin on its own supply, ask the host.

Related guides

• Swimming in Swedish lakes — the see-your-feet test and how beach notices work
• Swedish beaches — sea temperatures, upwelling and jellyfish on the same coasts
• Dogs in Swedish nature — leash rules and the other summer risks for dogs
• How 1177 works — calling the health line from a foreign phone
• Swimming safety — drowning risk and life jackets

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Facts verified 2026-09-25. Sources: SMHI, 1177, Livsmedelsverket (the Swedish Food Agency), Folkhälsomyndigheten, Havs- och vattenmyndigheten, Giftinformationscentralen, Länsstyrelsen — full register in this topic’s source file. Bloom status changes daily and is never published here; check SMHI’s map and the notice at the beach. Found something outdated? Tell us and we’ll fix it.